An organic electroluminescent (EL) device (hereinafter, briefly referred to as “organic EL device”) is generally composed of two opposite electrodes and at least one layer of an organic light-emitting compound inserted between these two electrodes. Electric charges are injected into the organic layer formed between the anode and the cathode to form electron hole pairs, so that the organic compound having fluorescent or phosphorescent characteristics generates light emission.
The research on the organic EL material started from 1950, beginning with the observation of Bernanose on an organic pigment-containing polymeric thin film to which high current and voltage was applied. In 1965, Pope et al. for the first time discovered the electroluminescent property of anthracene single crystal, which is the first example of the electroluminescence of organic compounds. In 1987, Tang et al. from Kodak Company discovered that even at a low voltage of 10V or less, an organic luminescent device, which is formed from an organic material and has laminated separate functional layers, can provide a high luminance of 1000 cd/cm2 or more.
In an organic material, the hole mobility is significantly higher than the electron mobility, so holes and electrons can be more effectively transported to the light-emitting layer when a hole transport layer and an electron transport layer are properly used. Additionally, when a balance between the hole density and the electron density in the light-emitting layer is achieved, the luminous efficiency can be improved.
When an electron and a hole recombine in an organic molecule, due to different manners of electron spin symmetry, two forms of excited state will occur. One is the form of a singlet excited state formed by a ground state electron with asymmetric spin, which releases energy in the form of fluorescence and then returns to the ground state; the other is the form of a triplet excited state formed by a ground state electron with symmetric spin, which releases energy in the form of phosphorescence and then returns to the ground state. According to theoretical speculation, the ratio of the singlet excited state to the triplet excited state caused by the recombination of electric charges is 1:3. If the energy of the singlet excited state is transferred to the triplet excited state for emitting phosphorescence, the internal quantum efficiency thereby may be close to 100%.
Generally, the phosphorescence host luminescent materials such as the carbazole ring compounds (e.g. CBP, etc.), and the phosphorescence guest luminescent materials such as a compound attached with iridium (Ir), platinum (Pt) or the like as a central metal atom, are widely used.